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Chemical and Physical Properties of Variable Stars in Globular Clusters
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
A study of 14 stars in seven different globular clusters has been undertaken. Ten of these stars are variable stars, while the four stars from NGC 7099 were used because they are near the RGB tip. Stellar parameters were determined by measuring the equivalent widths of absorption lines in the stellar spectra as well as other element abundances. The stellar parameters measured are: effective temperature (Teff), surface gravity (log(g)), metallicity ([Fe/H]) and the microturbulent velocity (vt). This was done using MOOG (Sneden 1973) and Kurucz (1981) model atmospheres. MOOG synth was used for the spectral lines O, Na, Mg, Al and Rb that were heavily blended or sensitive to other elemental abundances. The resulting stellar parameters have shown NGC 7099 4 to be a foreground star. Using available period data and spectral energy distribution fits, the remaining 13 stars were classified into five post-AGB stars and eight RGB/AGB stars. The "generation" for each star was determined using Na/O and Mg/Al plots. Three of the post-AGB stars are of particular interest. NGC 7492 V4 is a primordial star with a normal turnoff point at the tip of the AGB with high luminosity. It can be compared with NGC 7089 V5 and V6: potentially extreme stars and thus second-generation. V5 and V6 are likely to be He-rich due to having a much lower luminosity turnoff point from the AGB. This He-enrichment comes from being second-generation stars. Two stars had measurable rubidium lines that came out with the expected near-solar [Rb/Fe] ratio. Three stars with upper limits for Rb were found to be Rb-deficient.